Background: Surgical treatment of giant pituitary adenomas is difficult due to complicated dissection of the sellar area. The extent of tumor resection affects the efficacy of surgical treatment. This study is to investigate the efficacy of microsurgical treatment for giant pituitary adenoma and to analyze the relationship between treatment efficacy and tumor resection extent. Methods: A retrospective analysis was performed in 36 patients who received microsurgery to remove giant pituitary adenomas. The sizes of tumors before and after surgery were calculated with a novel method called the “platform-like volume calculation formula”. The relationships between extent of resection and the visual impairment recovery, and improvement of serum hormone level before and after operation were analyzed. Results: Two deaths were observed after surgery. And the gross and near-total resection was achieved in 8 cases, subtotal resection in 8 cases, mostly partial resection in 15 cases, and partial resection in 5 cases. The average resection rate was 72.8%. The resection rate of tumor with cavernous sinus invasion was significantly lower than those of patients without cavernous sinus invasion (P < 0.05). The improvement rate of hormone level in functional adenoma was 80.0%. Follow-up observations were carried out for 3 ~ 28 months in 25 cases. Visual improvement was observed in 64.0% of the cases. Conclusions: Microsurgical treatment can improve the visual impairment of the majority of cases and significantly decrease the serum hormone levels of functional adenoma. The extent of resection was significantly associated with cavernous sinus invasion.